This invention relates to semiconductor devices and particularly to the types which are fabricated on extremely thin and fragile semiconductor chips.
Certain types of semiconductor devices, particularly those operating in higher frequency ranges and with appreciable power ratings, are fabricated on extremely thin semiconductor chips typically having thicknesses of about two mils or less. The mil in this case is one-thousandth of an inch and roughly equal to twenty-five microns. Thus the chip thickness of interest in connection with this disclosre is about fifty microns or less. These dimensions are necessitated by the need for close spacing of PN junctions in order to obtain higher frequency performance and by the need to position the high current, heat generating rectifying junction, that is, the collector in a transistor, as close to a heat sink member as possible. Thus, in the case of transistors and even diodes of the high frequency and power type, a bulky semiconductor chip cannot be tolerated because of the increased thermal resistance. Accordingly, it has become the practice in the case of certain microwave power transistors to thin the wafers comprising the transistor chips to a thickness of fifty microns and less where possible. Obviously chips of this thickness are extremely fragile and difficult to process in the chip state. In particular, they are difficult to handle, to apply electrical leads to and to encapsulate.
A thin semiconductor device structure is disclosed in U.S. Pat. No. 3,777,227 wherein a thin web of material is supported by a thick rim of the semiconductor body. Such a web is fabricated by relatively complex and costly masking and etching steps. Moreover, the device disclosed therein utilizes the thick rim portion as an integral part of the electronic structure and is not concerned with heat dissipation.
Thus, an object of this invention is to enhance the strength and rigidity of thin semiconductor bodies facilely and with minimum cost.
Another object of this invention is to enable the fabrication of semiconductor devices in extremely thin chips without undue loss in yield from breakage and cracking. Another object of the invention is to improve the ability to handle such very small thin chips.